Charles Darwin is widely credited for his theories on evolution and natural selection. Samuel Butler's The Deadlock of Darwinism suggests that many of Darwin's ideas were not his own but instead taken from his predecessors. Butler states that the theories of evolution and natural were presented long before and that Darwin just tweaked the ideas and re-presented them. One of Butler's main points was that the theories of use and disuse had been proposed by John-Baptiste Lamarck long before Darwin published The Origin of Species. Butler argued that there was "no important difference" in the theories of Darwin and Lamarck except for the amount of influence of each part. I argue that this difference was very important and in fact explains the true genius of Darwin's work.
Lamarck believed that use and disuse was the primary means of evolution with a small part of it being due to spontaneous variation, whereas Darwin said spontaneous variation is the main reason behind variation with use and disuse being secondary. This is not a small difference in the theory because this slight change actually changes the theory by making it true. Darwin knew that Lamarck's theory of inheritance of acquired characteristics was wrong. We now know that changes that happened in an animal's life aren't passed on to their offspring. If you were to cut the tail off a dog like a Rottweiler, the offspring of the dog would still have a tail. We also now know that traits can only be passed on through genes, and genes cannot be affected by the environment. These and other examples show that Lamarck's theory was essentially wrong. Darwin corrected the theory by explaining that spontaneous variation is the key and essential factor behind variation. Any other version of this theory is false.
The theories that Butler claimed were stolen are like pieces of an intricate puzzle. What Darwin did is put the pieces together so you can see the whole picture. He took these theories, fixed any problems that he saw and put them together in a concise clear format that could be presented to the public. Darwin connects the theories and shows how cumulatively they can explain the variety of species on earth. Science has always been a cumulative field where the new generation builds off the theories of old. Copernicus was the first to suggest the heliocentric model of the solar system. Johannes Kepler corrected and added to Copernicus's theory by showing that the planets moved around the Sun in ellipses, instead of perfect circles as Copernicus had previously stated. Later Sir Isaac Newton was the first to propose that gravity was the reason behind the motion of the planets moving in ellipses. This is a classic example of how science is updated over time. This is essentially what Darwin did to many of the theories in question. Darwin changed the theories and put them in a context that made sense which is why he is till this day credited for his theories on Evolution.